Microphone Signals (Micip, Micin) - Wavecom GR64 GSM/GPRS Integrator's Manual

5.9.3

Microphone Signals (MICIP, MICIN)

MICIP and MICIN are balanced differential microphone input pins. These inputs are
compatible with an electret microphone. The microphone contains a FET buffer with
an open drain output, which is supplied with at 2.4V ±10% relative to ground by the
Wireless CPU as shown below.
Figure 17: Microphone input connections to the Wireless CPU
The input low-noise amplifier stage is constructed out of standard low-noise op
amps. External resistors set the gain of this stage.
The input gain is scaled by the input resistors to be around 15dB, which provides
optimal performance for many standard types of electret-microphones. An additional
gain adjustment can be made through an AT-command to the PGA which can be set
to high and low gain. The Wireless CPU provides a microphone bias at 2.4V, and can
supply at least 1mA of current.
Parameter
Input voltage full scale
Frequency response
AC input impedance
Output dc bias level
GR64 Integrators Manual
Page: 54/104
Conditions
max input gain
min input gain
-3dB cut-off
300–3400 Hz
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Min Typ Max
14 16 18
45 50 56
340 3400
10
2.16 2.4 2.64
Unit
mVrms
mVrms
Hz
kohm
V